LBL "Walking" microscope views atomic details in semiconductors

Device offers sharper look at nanometer-scale technologies

April 28, 1994

By Mike Wooldridge, MAWooldridge@lbl.gov

BERKELEY -- Using an ultrastable scanning tunneling microscope
(STM) that "walks" above surfaces, scientists at the Lawrence
Berkeley Laboratory have imaged never-before-seen features in
semiconductors, the electronic materials that are the basis for
computer chips.

LBL researchers have turned conventional STM design upside down by
developing a microscope that stands on three legs above a material
sample and walks itself to areas of interest. Many conventional
STMs hold a dime-sized sample atop finger-like prongs and examine
the sample from below.

The microscope legs of the new device are made out of a
piezo-ceramic material that bends slightly when electrified. By
flexing its legs, the microscope can jerk itself forward a few
nanometers at a time, or rotate in place.

LBL materials scientists Eicke Weber, Jun-Fei Zheng, and Miquel
Salmeron have used the STM to produce the first atom-by-atom images
of an indium gallium arsenide/gallium arsenide interface, one of
the more common architectures found in the semiconductors of
lasers. They have also produced the first images of silicon donor
atoms, the electron-rich impurities that give semiconductor
crystals their special electrical properties. Both sets of images
have been published in the Physical Review Letters.

"We are entering an exciting new phase in materials science," Weber
says. "Until now, people have had to use indirect methods such as
spectroscopy to explore these atomic features. Now we can use a
microscope to look at them directly. We'll be able to use the new
information to improve the way semiconductors are created."

The ability to investigate semiconductors atom-by-atom is
especially important as production begins of electronic devices on
a nanometer scale. At such a small scale, the precise placement of
individual atoms is critical since the size of the atoms themselves
begin to set the limits of performance.

Since its invention in 1981, the STM has been the most important
tool for studying the atomic surface structures of metals and
semiconductors. The device images the arrangements of atoms by
moving an electrified metal tip a few atoms away from a material's
surface. At such a close range, electrons will spontaneously jump
back and forth between the tip and the sample material, a
phenomenon known as tunneling. By measuring the rate of electron
tunneling as it scans the material, an STM can map the topography
of the material's surface.

In the past, however, researchers were unable to bring an STM tip
close enough to a semiconductor surface to image features such as
donor atoms. This is because mechanical instabilities and slight
temperature fluctuations -- which cause the microscope's different
parts to expand and contract -- can smash a tip of a microscope
into a sample material if it is too close.

In addition to being able to walk, the new STM can get its tip much
closer to a surface because it is more compact, and therefore less
susceptible to mechanical vibrations. It also has a symmetrical
design, so that shape changes in its different parts offset one
another. With such an ultrastable STM, the donor atoms that
appeared as faint shadows in past images now emerge as glowing
spheres.

LBL is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.